How Muscle Fiber Recruitment Affects Running Performance

Train to get your muscles to do what you want

That's a term reserved for football players, weight lifters, or Charles Atlas when he kicks sand in the face of a skinny runner on the beach.

But the reality is that the human body has more than 600 muscles. We runners use most of them. And training sessions designed to recruit and strengthen our running muscles are the key to achieving top fitness.

"If you teach your body to recruit muscle fibers powerfully and efficiently, you'll run faster and increase your VO2 max," says Beaverton, Ore., coach and exercise scientist Tom Schwartz, whose advice at www.therunzone.com has helped scores of runners. "The difference between a novice and elite runner is that the novice's muscles haven't been trained to generate enough sustained force."

So what does this mean for us distance runners? Are we to spend summers at Muscle Beach, pumping iron like the tanned, granite-chiseled bodybuilders who populate Hollywood action films and gave California its governor?

That's a negative. Instead, we need to incorporate workouts that target the muscle fibers responsible for endurance and speed. The good news is that, without realizing it, most of us employ some of these workouts already. The better news is that a few training tweaks and a couple added workouts can significantly improve our performances.

What's a muscle fiber?

A muscle "fiber" is what we call a muscle cell. Muscle cells contract and relax in response to messages from our central nervous system.

Our bodies have three different kinds of muscle. Cardiac muscle is found in our heart. Smooth muscle controls involuntary functions like digestion and blood pressure. And skeletal muscle moves our body – making it the muscle we focus on when training.

Skeletal muscle accounts for more than a third of our body mass, and it can be broken down into three distinct types of fibers:

Slow-twitch fibers contract more slowly and less forcefully than Type II fibers. Distance runners love them because they're teeming with mitochondria, myoglobin and capillaries – ingredients that ensure the steady supply of oxygen and energy we need to run aerobically.

Intermediate fibers share many of the aerobic characteristics of slow-twitch fibers, but they can contract faster and with more force. This makes them perfect for prolonged fast running, the kind demanded for 800m and 1500m races.

Fast-twitch fibers are the speed demons of muscle cells. They contract fastest and most forcefully of the fiber types. But they're also limited by an inability to function aerobically, providing only short bursts of anaerobically fueled running.

Every skeletal muscle (calf, hamstring, etc.) contains fibers of all three types. But not all runners have the same percentage of each. Elite distance runners have high percentages of slow-twitch and intermediate fibers. Sprinters are rich in fast-twitch. Genetics determines the percentage of fiber types in our legs, but training can alter how these fibers function.

How do muscle fibers work?

Most of us take the act of running for granted. We "tell" our bodies to start running. Tell them when to run faster or slower. Tell them to blast off the start line or to kick the final 100 meters of a race. But understanding how our bodies use nerve and muscle to create motion provides the foundation for creating a better training program.

It all begins in our central nervous system. Messages originate there, before being sent via nerves to targeted muscles throughout our body. We call a nerve cell and the muscle fibers it controls a "motor unit." A motor unit can contain more than a thousand muscle fibers (all of the same type: slow-, intermediate, or fast-twitch), and these fibers contract in unison. Groups of motor units work together to contract an entire muscle.

For one muscle to contract successfully, another muscle must relax. Consider your upper arms. For your biceps to flex, your triceps must relax, and vice versa. Many pairs of muscles contract and relax to create our running stride. Improved coordination of this contraction and relaxation makes us more efficient runners.

The muscle fiber ladder

When we run, we recruit muscle fibers in a "ladder." Instead of using all our fiber types immediately, we begin by recruiting slow-twitch, which forms the bottom rung of our ladder. If the force required for our run increases – either because we pick up the pace or encounter resistance (a hill, etc.) – we move up the ladder, recruiting intermediate fibers to supplement our slow-twitch ones. At maximum workloads, we add fast-twitch fibers to the mix, reaching the top rung.

"As long as fatigue is not a factor, muscle fiber recruitment is always force-dependent," says Schwartz, whose athletes include two-time national masters indoor 3,000m champ Andrew Duncan. "If you need more force, you recruit up the fiber ladder."

When fatigue is a factor, it too can act as a stimulus for moving up the ladder. Long runs can deplete glycogen in slow-twitch fibers to the extent that intermediate fibers are recruited.

Schwartz speculates that we recruit the least amount of fibers necessary to get the job done. This means that a fit runner uses fewer slow-twitch fibers to run 8:00/mile pace than to run 6:00/mile pace. The rate at which our fibers contract also changes. "When you get to about 60 percent of all the fibers you can use," says Schwartz, "you begin using them at a faster rate."

The Fiber Ladder chart below gives approximations for when novice and fit runners can expect to move up the ladder, based upon percentage of VO2 max. It also indicates the points at which both maximum recruitment and maximum contraction rate occur for each fiber type. This information is vital when setting pace goals for workouts.

THE MUSCLE FIBER LADDERCreated by Tom Schwartz

Runner's Fitness Level

Very Low

Low

Medium

High

Very High

Fiber Type

Intial Recruitment Threshold of Muscle Fibers, as a Percentage of VO2 Max

Type I

>0

>0

>0

>0

>0

Type IIa

50.8

54.0

57.8

62.0

67.0

Type IIx

67.7

72.0

77.0

82.7

89.3

Fiber Type

Peak Recruitment Threshold of Muscle Fibers, as a Percentage of VO2 Max

Type I

56.8

60.5

64.7

69.4

75.8

Type IIa

75.8

80.6

86.2

92.6

100.0

Type IIx

101.0

107.5

114.9

123.5

133.3

Fiber Type

Maximum Stimulation Rate of Muscle Fibers, as a Percentage of VO2 Max

Type I

63.1

67.2

71.8

77.2

83.3

Type IIa

84.2

89.6

95.8

102.9

111.1

Type IIx

112.2

119.5

127.7

137.2

148.1

How do we train muscle fibers?

There's no aspect of muscle fiber recruitment and activation that can't be improved through training. For our purposes, we'll focus on five areas of muscle fiber training:

When we run, a micro-Darwinian "survival of the fittest" takes place in our muscle fibers. Fibers contain small myofilaments called myosin and actin; weaker myofilaments are damaged through training. Our body responds by replacing these damaged myofilaments with stronger ones, leaving us with fibers that can better handle the stress of running.

Also, as we increase the intensity and volume of our running, the number of these myofilaments increases, causing muscle fibers to swell. It's this increase in the size of muscle fibers (called "hypertrophy") that leads to visible enlargement of our muscles. The combination of stronger and larger fibers allows our muscles to produce more force, while simultaneously increasing their resistance to damage and fatigue.

MAXIMUM RECRUITMENT

Knowing that we can strengthen our muscle fibers, it stands to reason that we'd want to strengthen all of them. Yet many runners don't. Distance runners who focus solely on volume miss the opportunity to strengthen intermediate and fasttwitch fibers. Sprinters who eschew intervals fail to develop intermediate fibers to their full potential.

Recruiting and then strengthening all three types of muscle fibers is essential to achieving our best running. Of course, we have to be sensible in how we structure this training. "We can't train fast-twitch every day," says Schwartz, noting that fibers have only so much adaptive energy. "As for the lower-end fibers, you can't stimulate them completely with short, high-quality workouts. They require a combination of low intensity and long duration to get maximum training effect."

RECRUITMENT PATTENRNS

Boyd R. McCandless, a psychologist, has written, "The ultimate goal of physical motor development is to make the human organism as independent of gravity as is possible within the limitations of the human body."

As children, we learned gross muscle control through the process of trial and error, fueled by our internal desire to master movement and defy gravity. We runners need to continue this practice, experimenting with a wide range of pace, effort, force and technique. In doing so, we create adaptations that improve our body's function in the following areas:

The sequence in which we recruit muscle fibers;Coordination of fiber recruitment at specific paces and fatigue levels;Ability to target running-specific fibers; andEnlistment of the full range of useful fibers.

REDUCED INHIBITION

When one muscle contracts, another must relax. But if relaxation isn't complete (e.g., if we flex our triceps while trying to flex our biceps), then movement is inhibited. By improving our ability to coordinate contraction and relaxation, we reduce inhibition.

"Muscles don't know what the heck to do," says Schwartz. "Opposing muscles aren't in sync. In order to lift your knees, the opposing muscle must relax. What's important is reducing this inhibition. This reduces our energy expenditure, so we can run farther faster."

FIBER CONVERSION

There's an ongoing debate as to whether one type of fiber can fully convert to a different type. But there's no doubt that faster fibers can be trained to take on characteristics of slower fibers – vital for distance runners looking to maximize both force and endurance.

"With endurance training," says Schwartz, "you can transform even the very fastest fibers – from a chemical point of view – to endurance fibers. If we take someone who has never trained before, and then train his right leg for eight weeks and not the left, the sprint fibers in the trained leg will look and act like the slow fibers of the untrained leg."

Schwartz points out that there's a limit to this conversion. If Usain Bolt were to train his fast-twitch fibers for endurance, those fibers would increase their mitochondrial count, but they would never achieve a count equal to that found in an elite distance runner's slow-twitch fibers.

Workouts

Understanding muscle fibers allows us to design workouts that target specific fiber types, train them at the correct intensity and for the proper duration, and ensure maximum adaptation.

The following workouts are integral parts of a well-rounded training program, no matter what your target race distance. They're broken down by targeted fibers, description of the workout, and desired effect. Also, each breakdown includes comments from Schwartz on some of the physiological principles in play. Again, all are key to maximizing your performance at common race distances. Emphasize some more than others to best meet the demands of your target event (more workouts that target slow-twitch fibers for marathons, more that target intermediate fibers for 5Ks, etc.)

THE LONG RUN

FIBER: Slow-Twitch, Intermediate

WORKOUT: The long run is the backbone of our endurance training. Run once or twice per week, its length varies depending upon the runner's fitness, philosophy and race goals. Arthur Lydiard had his runners do a 22-mile grueling hill run once a week. ASICS Aggies coach Joe Rubio recommends two long runs a week, accounting for 20-25 percent and 15 percent of overall volume. We should keep in mind that many desirable adaptations occur after the 90-minute mark. The pace is generally conversational, with an emphasis on quantity, not quality.

EFFECT: By running long, we provide the necessary stress to ensure maximum adaptation of our slow-twitch fibers. We weed out weak myofilaments, replacing them with stronger ones. And as fatigue sets in, we begin recruiting intermediate fibers, improving their endurance capacity and coordinating their use alongside the less force-generating slow-twitch fibers.

SCHWARTZ: "We're building stronger slow-twitch fibers, so we'll have to use less of their maximum capacity at a given speed. That saves on both fuel and oxygen costs. Also, a slow-twitch fiber can fire faster by 50 percent or more after a marathon-training program."

LONG INTERVALS

FIBERS: Slow-Twitch, Intermediate

WORKOUT: These repetitions range from 2 to 5 minutes (up to 6 minutes for highly fit runners) and can be run on the track, road, or trail. The pace mimics race efforts from 3K to 15K. Recovery can equal the duration of the repetition during early sessions, then is shortened as we get more fit.

EFFECT: Strength is increased in both slow-twitch and intermediate fibers as weak myofilaments are replaced with stronger ones. Slow-twitch fibers are forced to work at top contraction speed. And since all our slow-twitch and intermediate fibers are recruited, we develop a more efficient working relationship between the two fiber types.

SCHWARTZ: "[Most runners] don't run longer than 3 to 5 minutes because our speed would drop, at which point we're training more slow-twitch and not recruiting intermediate to the same level. Moderate length intervals focus on strength and power."

SHORT INTERVALS

FIBERS: Intermediate, Fast-Twitch

WORKOUT: These shorter repetitions last from 30 to 90 seconds and can be run on the track, grass, or trails. Pace is based upon race times at distances from 800m to 3K. effect: Both intermediate and fast-twitch fibers are strengthened. Our two fastest fibers learn to interact more efficiently by reducing activation of unnecessary fibers.

SCHWARTZ: "Short intervals help us develop a sense of rhythm for our race – both neuromuscular and cognitive. Our bodies rely on sensory feedback for running pace: the pressure of the ground, muscle tension, and visually what we're seeing."

LONG HILL REPEATS

FIBERS: Slow-Twitch, Intermediate workout: Lasting from 30 to 90 seconds, these repetitions are performed on a moderately steep hill. Four to 10 repetitions are sufficient, with at least 2 minutes recovery for shorter reps and up to 5 minutes for longer ones. Schwartz suggests that the pace should be equivalent to what we could run all-out for 3 minutes up the hill. I instruct the runners I coach to find an effort level that allows them to finish the session with gas in the tank; they should hypothetically be capable of running an additional one to two reps if the workout called for it.

EFFECT: Since it's force – not speed – that builds strength, this workout is more effective than intervals on the track, trails, etc., at strengthening intermediate fibers. By keeping our effort at the correct level, we create maximum stress on our slow-twitch and intermediate fibers. Running too fast recruits the big boys – our fast-twitch fibers – to do the heavy lifting, which in turn leads to rapid fatigue, less work for the targeted fibers, and a longer recovery period before we can train hard again.

SCHWARTZ: "An intermediate fiber combines moderately high production of power with a medium-length duration of running. While a single all-out hill for 60 seconds will temporarily fatigue an intermediate fiber, it won't deplete the glycogen enough for full effect. You must deplete about 75 percent capacity of glycogen content of the muscle fiber type. Long hill reps accomplish this."

SHORT HILL REPS

FIBERS: Fast-Twitch workout: Short, steep uphill repetitions lasting less than 10 seconds. Our effort should reach 90-95 percent of maximum – not an all-out sprint, but close. Two to three minutes is a good recovery period.

EFFECT: This workout strengthens all our fiber types, but we use it to focus on fast-twitch, which requires a 90-95 percent effort to become activated. Because the range of motion required to sprint up a hill is greater than on the flat, we recruit a fuller complement of all fiber types. This is also one of the best workouts for reducing inhibition, as it curtails the activation of fibers that don't contribute to our speed.

SCHWARTZ: "Force, not speed, determines how many motor unit fibers are activated. Even though you turn your legs over faster on the flat, speed doesn't determine activation of fibers. Force does. That's why hill sprints are great."

TEMPO

FIBERS: Slow-Twitch, Intermediate

WORKOUT: Über coach Jack Daniels has described tempo as a "comfortably hard" effort – about 25 to 30 seconds slower than 5K race pace – for 20 minutes. Pete Pfitzinger alters this rule for super-fit marathoners, suggesting up to 9 miles at half marathon to marathon pace.

EFFECT: Slow-twitch fibers reach maximum recruitment and contraction speed at about 80 percent VO2 max – in other words, at tempo pace. This means that all slow-twitch fibers are strengthened. Intermediate fibers will also be recruited, helping our bodies establish recruitment patterns for the two fiber types at half marathon and marathon paces.

SCHWARTZ: "A runner who doesn't have much leg speed – who doesn't possess much fast-twitch fiber – won't want to do much speed work. The only way for that runner to prosper is to do more work at around 80 percent VO2 max."

PROGRESSION

FIBERS: Slow-Twitch, Intermediate

WORKOUT: We begin the run at an easy effort, then drop our pace 10-15 seconds each mile until we can't lower the pace again without achieving a race-type stress. Most runners top out at about 5K race pace.

EFFECT: As with tempo, progression runs achieve maximum activation of slow-twitch fibers. They incrementally recruit intermediate fibers, improving coordination between the fiber types. Some fast-twitch recruitment during the last miles might aid fiber conversion.

SCHWARTZ: "A runner at 40 percent VO2 max at 8:00/mile pace continues to add slow-twitch fibers until he reaches maximum recruitment at about 7-minute pace, or 60 percent VO2 max. At that point, 'overlap' occurs, with intermediate fibers jumping in. All the way up to 85 percent VO2 max, slow-twitch fibers work at a progressively faster rate, and we incrementally increase the stress on our intermediate fibers for a sustained time. We're improving endurance as well as strength – although mostly endurance."

TECHNIQUE DRILLS

FIBERS: All Fibers

WORKOUT: Also called "form drills," this workout involves variations of skipping, bounding, marching, springing, fast knee lifts, quick foot movement, and any number of activities meant to mimic, exaggerate, or strengthen our running motion. For best effect, 60m-80m strides are run between drills, and a short distance run is completed afterward.

EFFECT: Drills like skipping and butt kicks mimic the effects of dynamic stretching. Bounding and springing are more plyometric, affecting both recruitment patterns and strength. Marching and fast knees reduce inhibition. Alternating each drill with a stride helps to integrate the desired effect into our running mechanics. A short run of 5 to 7 miles after the drills further hardwires these neuromuscular gains into our stride.

SCHWARTZ: "Drills help us run relaxed and through a big range of motion by improving coordination and reducing neural inhibition. Plyometric drills affect motor-neural sequencing and firing, what many people call 'recoil' or 'elastic recoil.' Some smaller elements of running's neural component are improved, such as Golgi tendon stretch reflex."

We runners tend to view developing our cardiovascular systems as the key to achieving better fitness. But the truth is that high-octane fuel can't turn a Prius into a Porsche. And no amount of oxygen – or Gatorade or PowerBars or willpower – will produce better running performances until we've built a better running body.

DETERMINING VO2 MAX PACE

Since most athletes don't have access to sophisticated VO2 testing, Tom Schwartz suggests using a field test. After warming up, run a time trial of 2400m (2,000m for less fit runners) on a 400m track.

Your pace for the time trial is comparable to your running speed at VO2 max. For example, a runner who completes his six-lap time trial in 7:50 has a pace of roughly 5:00/mile at VO2 max.

To calculate your VO2 max pace:

» Run a 2400m time trial (2,000m if you're relatively inexperienced or returning to real training).

» Divide your finish time by 6 to calculate per-lap pace.

» Multiply per-lap pace by 4 to establish mile pace at 100% VO2 max.

According to Schwartz, your mile pace for the time trial is your maximum aerobic speed (MAS). Schwartz suggests using MAS as a basis for determining pace during certain workouts. His guidelines:

Distance – 70% of MAS Tempo – 80% of MAS

Long Intervals – 90% of MAS Short Intervals – 100% of MAS

Senior writer PETE MAGILL holds three American age-group records at distances from 3,000m to 10,000m.